use std::{
    iter::{IntoIterator, Iterator},
    ops::Drop,
};

type Link<T> = Option<Box<Node<T>>>;

pub struct List<T> {
    head: Link<T>,
}

struct Node<T> {
    value: T,
    next: Link<T>,
}

impl<T> List<T> {
    pub fn new() -> Self {
        List { head: None }
    }

    pub fn push(&mut self, value: T) {
        let new = Box::new(Node {
            value,
            next: self.head.take(),
        });

        self.head = Some(new);
    }

    pub fn pop(&mut self) -> Option<T> {
        self.head.take().map(|node| {
            self.head = node.next;
            node.value
        })
    }

    pub fn peek(&self) -> Option<&T> {
        self.head.as_ref().map(|node| &node.value)
    }

    pub fn peek_mut(&mut self) -> Option<&mut T> {
        self.head.as_mut().map(|node| &mut node.value)
    }

    pub fn iter<'a>(&'a self) -> Iter<'a, T> {
        Iter(self.head.as_deref())
    }

    pub fn iter_mut<'a>(&'a mut self) -> IterMut<'a, T> {
        IterMut(self.head.as_deref_mut())
    }
}

impl<T> Drop for List<T> {
    fn drop(&mut self) {
        let mut head = self.head.take();
        while let Some(_node) = head {
            head = self.head.take();
            // node drop here, and node.next is Empty, not rec
        }
    }
}

pub struct IntoIter<T>(List<T>);

impl<T> Iterator for IntoIter<T> {
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        self.0.pop()
    }
}

impl<T> IntoIterator for List<T> {
    type Item = T;

    type IntoIter = IntoIter<Self::Item>;

    fn into_iter(self) -> Self::IntoIter {
        IntoIter(self)
    }
}

pub struct Iter<'a, T>(Option<&'a Node<T>>);

impl<'a, T> Iterator for Iter<'a, T> {
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        self.0.map(|node| {
            self.0 = node.next.as_deref();
            &node.value
        })
    }
}

pub struct IterMut<'a, T>(Option<&'a mut Node<T>>);

impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;

    fn next(&mut self) -> Option<Self::Item> {
        self.0.take().map(|node| {
            self.0 = node.next.as_deref_mut();
            &mut node.value
        })
    }
}

mod test {
    use super::List;
    #[test]
    fn iter() {
        let mut list = List::new();
        list.push(1);
        list.push(2);
        list.push(3);

        let mut iter = list.iter();
        assert_eq!(iter.next(), Some(&3));
        assert_eq!(iter.next(), Some(&2));
        assert_eq!(iter.next(), Some(&1));
    }

    #[test]
    fn into_iter() {
        let mut list = List::new();
        list.push(1);
        list.push(2);
        list.push(3);

        let mut iter = list.into_iter();
        assert_eq!(iter.next(), Some(3));
        assert_eq!(iter.next(), Some(2));
        assert_eq!(iter.next(), Some(1));
        assert_eq!(iter.next(), None);
    }

    #[test]
    fn iter_mut() {
        let mut list = List::new();
        list.push(1);
        list.push(2);
        list.push(3);

        let mut iter = list.iter_mut();
        assert_eq!(iter.next(), Some(&mut 3));
        assert_eq!(iter.next(), Some(&mut 2));
        assert_eq!(iter.next(), Some(&mut 1));
    }

    #[test]
    fn basics() {
        let mut list = List::new();

        // 检查空列表行为正确
        assert_eq!(list.pop(), None);

        // 填充列表
        list.push(1);
        list.push(2);
        list.push(3);

        // 检查通常的移除
        assert_eq!(list.pop(), Some(3));
        assert_eq!(list.pop(), Some(2));

        // 推入更多元素来确认没有问题
        list.push(4);
        list.push(5);

        // 检查通常的移除
        assert_eq!(list.pop(), Some(5));
        assert_eq!(list.pop(), Some(4));

        // 检查完全移除
        assert_eq!(list.pop(), Some(1));
        assert_eq!(list.pop(), None);
    }

    #[test]
    fn peek() {
        let mut list = List::new();
        assert_eq!(list.peek(), None);
        assert_eq!(list.peek_mut(), None);
        list.push(1);
        list.push(2);
        list.push(3);

        assert_eq!(list.peek(), Some(&3));
        assert_eq!(list.peek_mut(), Some(&mut 3));
    }
}
